Haven't tried this myself yet, but the first thing that comes to my mind: do you need the libhoudini stuff on a smartphone at all? From what I understood, this is needed for anbox running on Intel/AMD platform (i.e. a PC).

About the read-only filesystem I would guess that you have to remount the android image as read-write, not the UT root FS.

Edit: wanted to try it out but the opengapps repositories are disabled by github due to excessive use of resources...

So I was able to add the gapps to the android.img on my PC, then transferred the image to the phone (don't forget to anbox-tool disable before the transfer and anbox-tool enable after it's finished) and saw the Play Store icon on the app scope

The apps that said they would not run properly without gapps now said I need to update my Play Services (by backup only contained the gapps from Dec. 2017). That unfortunately failed. Also opening the Play Store didn't work. Guess it also found the play services to be outdated and then failed to update them.

I guess it only makes sense when using up-to-date opengapps (once their repos are back up).

With current opengapps pico it's similar to my microG experience: everything gets more unstable and apps seem to start but then close unexpectedly.

For now I'm back to unmodified anbox.

Maybe this is device related. I'm on Fairphone 2 and I've read about frequent crashes that don't happen on other phones. Maybe better luck with those. But then, maybe I overlooked something and someone with an FP2 has more luck (or time to dig deeper...).

Well did you remember to
"sudo anbox-tool disable
sudo mount -o rw,remount /"

like in the script?(and reverse afterwards)

I never got opengapps to work but I think to edit the system I ended up copying the android.img to another directory, running unsquashfs on it, modifying it, resquashing it (mksquashfs) and then overwrite the original image.